8 ms·

Opposing arrows of time can theoretically emerge from certain quantum systems

96 points by jnord 2y ago

chrisjj 2y ago
Clickbait title. The body reports no evidence.
- dang 2y ago
  Ok, we've replaced it with a perhaps more representative phrase from the first paragraph.
  - chrisjj 2y ago
    Sorry to have been sufficiently specific. I am critical of the article, not the post. "Physicists uncover evidence of two arrows of time emerging from the quantum realm"
    - chrisjj 2y ago
      > Sorry to have been sufficiently specific s/sufficiently/insufficiently/
luxuryballs 2y ago
“suggesting that time’s arrow may not be as fixed as we experience it” this is what they mean by stretching spacetime right?
andrewfromx 2y ago
So if you visualize milk spilling on a table in reverse that sense of that being fake or wrong or not possible is just your human point of view? Better to think of that like you are seeing a pendulum. Moving time backwards isn't impossible. It's just improbably?
- klysm 2y ago
  That might be a bad example due to gravity
someothherguyy 2y ago
https://www.nature.com/articles/s41598-025-87323-x https://www.nature.com/articles/s41598-025-87323-x
MattPalmer1086 2y ago
I have never understood why physicists get so hung up on the arrow of time and entropy. If you have a casual system, then statistically, things will tend to become more disordered over time, because there are just more ways to be disordered than ordered. Lots of clever people seem to agonise about this, but I don't see any problem. What am I missing?
- DiogenesKynikos 2y ago
  That the laws of physics are symmetric under time reversal[0], so any statement you make about how things should evolve "over time" also holds if you go backwards in time. The apparent arrow of time is commonly thought to have to do with initial conditions. Entropy was low right after the Big Bang, creating an asymmetry between the two time directions. 0. Technically, the laws of physics are symmetric under stimultaneous inversion of time, flipping of all charges, and a mirror reflection of space.
- JohnKemeny 2y ago
  What is time?
  - GenerocUsername 2y ago
    Our brains operate on the principle entropy, therefore we experience time as the macro evolution of the universe along the dimension of increasing entropy. Time itself does not exist, we merely experience it because it is how our consciousness experiences reality
    - geysersam 2y ago
      Do you mean to say that our experience of time depends on what we see happening around us? If I place you in a room with nothing but a video tape playing backwards, would you experience time going backwards? That doesn't sound plausible.
      - garciasn 2y ago
LinuxAmbulance 2y ago
"...suggesting that time’s arrow may not be as fixed as we experience it." That sentence is doing a lot of heavy lifting in this article.
pizza 2y ago
The Minkowski metric is Δs = sqrt(-Δt^2 + Δx^2 + Δy^2 + Δz^2) One aspect of this is that, if you sub `t -> -t'`, that's just as good a solution too. Which would suggest any solution with a positive time direction can have a negative time direction, just as easily. Is this widely assumed to be true, or at least physically meaningful? There's also Wick rotations, where you can sub `t -> it'`, and then Minkowskian spacetime becomes Euclidean but time becomes complex-valued. Groovy stuff. I'm not much of a physics buff but I loved reading Julian Barbour's The Janus Point for a great treatment of the possibility of negative time. The craziest thing I've seen though is the suggestion that an accelerating charge, emitting radiation that interacts with the charge itself and imparts a backreacting force on the charge, supposedly has solutions whose interpretation would suggest that it would be sending signals back in time. [0] [0] https://en.wikipedia.org/wiki/Abraham–Lorentz_force https://en.wikipedia.org/wiki/Abraham–Lorentz_force
- echelon 2y ago
  How much of this is just the mathematical model permitting such things? Has any of this been experimentally fit to reality? > sending signals back in time. First encode stock movements. Then technological discoveries. If it works like many-worlds, then even NP hard problems. With sufficient bandwidth and time delta, you'd also have to hope that the future isn't sending anything nefarious that could lead to your demise. A future adversary could final destination you pretty easily and use you as a pawn to enrich itself, and there's not much you could do except stop listening - but it'd know you planned that too. A future adversary would be the scariest adversary.
  - vjerancrnjak 2y ago
    People play with models like this and get to white holes.
    - echelon 2y ago
      Maybe the reason we don't see aliens is that the first ones to break physics restart the universe [1]. Fragile universe hypothesis [2]. [1] eg. nucleate vacuum collapse [2] cf. fragile world hypothesis
layer8 2y ago
From the paper’s conclusion: “Our findings are consistent with the second law of thermodynamics and emphasise the distinction between the concepts of irreversibility and time-reversal symmetry. Once the arrow of time and a particular low entropy initial condition at have been chosen, then the von Neumann entropy will increase forward in time from the temporal origin. However, a different choice of the arrow of time would have implied the same dynamics. The Markov approximation applied to the time-reversed evolution leads likewise to the same dissipation and entropy increase. Consequently any thermal equilibrium state for a forward-running trajectory is also an equilibrium thermal state for any time-reversed trajectory, and entropy increases in both directions: the system thermalises into both time directions. […] “Furthermore, we speculate that these results may reflect on the cosmological arrow of time. In fact, the natural assumption that the universe was dissipative from time zero onwards would suggest that a model of it would rely on the Markov approximation performed at the moment of the Big Bang. If so, this would imply that two opposing arrows of time would have emerged from the Big Bang, which would account in turn for the maintenance of time-reversal symmetry despite the ensuing dissipative nature of the universe. We would happen to live in one of them, where dissipation and entropy increase are common experience, but unaware of the existence of the other alternative possibility.”
hoppp 2y ago
When can we send information back in time? I just want to send myself some stock market data. Im pretty sure I can encode all information I need in 1 qbit(buy or sell).hehe
deleted 2y ago
[deleted]
cjfd 2y ago
The standard answer is that apparently the initial state of our universe had a low entropy and that has been increasing ever since.
- throwaway314155 2y ago
  That's the standard answer to a question you're not responding to, as far as I can tell.
steveBK123 2y ago
Better headline - physicists discover you could theoretically "Do Tenet".
pingou 2y ago
It seems to me that time could run backward and it would not make any difference to an observer, and the universe could alternate between running backward or forward and that would be the same to us, cause and effects would be reversed but it would only be visible to an outside observer, that has somehow time running in a different direction.
- pwr22 2y ago
  There's a Philip K Dick story that covers this type of thinking, "Your appointment will be yesterday".
janpmz 2y ago
How long will it take for people to realize, they are looking at a contradiction?

96 points by jnord 2y ago

chrisjj 2y ago

Clickbait title. The body reports no evidence.

dang 2y ago
Ok, we've replaced it with a perhaps more representative phrase from the first paragraph.
- chrisjj 2y ago
  Sorry to have been sufficiently specific. I am critical of the article, not the post. "Physicists uncover evidence of two arrows of time emerging from the quantum realm"
  - chrisjj 2y ago
    > Sorry to have been sufficiently specific s/sufficiently/insufficiently/

luxuryballs 2y ago

“suggesting that time’s arrow may not be as fixed as we experience it” this is what they mean by stretching spacetime right?

andrewfromx 2y ago

So if you visualize milk spilling on a table in reverse that sense of that being fake or wrong or not possible is just your human point of view? Better to think of that like you are seeing a pendulum. Moving time backwards isn't impossible. It's just improbably?

klysm 2y ago
That might be a bad example due to gravity

someothherguyy 2y ago

https://www.nature.com/articles/s41598-025-87323-x https://www.nature.com/articles/s41598-025-87323-x

MattPalmer1086 2y ago

I have never understood why physicists get so hung up on the arrow of time and entropy. If you have a casual system, then statistically, things will tend to become more disordered over time, because there are just more ways to be disordered than ordered. Lots of clever people seem to agonise about this, but I don't see any problem. What am I missing?

DiogenesKynikos 2y ago
That the laws of physics are symmetric under time reversal[0], so any statement you make about how things should evolve "over time" also holds if you go backwards in time. The apparent arrow of time is commonly thought to have to do with initial conditions. Entropy was low right after the Big Bang, creating an asymmetry between the two time directions. 0. Technically, the laws of physics are symmetric under stimultaneous inversion of time, flipping of all charges, and a mirror reflection of space.
JohnKemeny 2y ago
What is time?
- GenerocUsername 2y ago
  Our brains operate on the principle entropy, therefore we experience time as the macro evolution of the universe along the dimension of increasing entropy. Time itself does not exist, we merely experience it because it is how our consciousness experiences reality
  - geysersam 2y ago
    Do you mean to say that our experience of time depends on what we see happening around us? If I place you in a room with nothing but a video tape playing backwards, would you experience time going backwards? That doesn't sound plausible.
    - garciasn 2y ago

LinuxAmbulance 2y ago

"...suggesting that time’s arrow may not be as fixed as we experience it." That sentence is doing a lot of heavy lifting in this article.

pizza 2y ago

The Minkowski metric is Δs = sqrt(-Δt^2 + Δx^2 + Δy^2 + Δz^2) One aspect of this is that, if you sub `t -> -t'`, that's just as good a solution too. Which would suggest any solution with a positive time direction can have a negative time direction, just as easily. Is this widely assumed to be true, or at least physically meaningful? There's also Wick rotations, where you can sub `t -> it'`, and then Minkowskian spacetime becomes Euclidean but time becomes complex-valued. Groovy stuff. I'm not much of a physics buff but I loved reading Julian Barbour's The Janus Point for a great treatment of the possibility of negative time. The craziest thing I've seen though is the suggestion that an accelerating charge, emitting radiation that interacts with the charge itself and imparts a backreacting force on the charge, supposedly has solutions whose interpretation would suggest that it would be sending signals back in time. [0] [0] https://en.wikipedia.org/wiki/Abraham–Lorentz_force https://en.wikipedia.org/wiki/Abraham–Lorentz_force

echelon 2y ago
How much of this is just the mathematical model permitting such things? Has any of this been experimentally fit to reality? > sending signals back in time. First encode stock movements. Then technological discoveries. If it works like many-worlds, then even NP hard problems. With sufficient bandwidth and time delta, you'd also have to hope that the future isn't sending anything nefarious that could lead to your demise. A future adversary could final destination you pretty easily and use you as a pawn to enrich itself, and there's not much you could do except stop listening - but it'd know you planned that too. A future adversary would be the scariest adversary.
- vjerancrnjak 2y ago
  People play with models like this and get to white holes.
  - echelon 2y ago
    Maybe the reason we don't see aliens is that the first ones to break physics restart the universe [1]. Fragile universe hypothesis [2]. [1] eg. nucleate vacuum collapse [2] cf. fragile world hypothesis

layer8 2y ago

From the paper’s conclusion: “Our findings are consistent with the second law of thermodynamics and emphasise the distinction between the concepts of irreversibility and time-reversal symmetry. Once the arrow of time and a particular low entropy initial condition at have been chosen, then the von Neumann entropy will increase forward in time from the temporal origin. However, a different choice of the arrow of time would have implied the same dynamics. The Markov approximation applied to the time-reversed evolution leads likewise to the same dissipation and entropy increase. Consequently any thermal equilibrium state for a forward-running trajectory is also an equilibrium thermal state for any time-reversed trajectory, and entropy increases in both directions: the system thermalises into both time directions. […] “Furthermore, we speculate that these results may reflect on the cosmological arrow of time. In fact, the natural assumption that the universe was dissipative from time zero onwards would suggest that a model of it would rely on the Markov approximation performed at the moment of the Big Bang. If so, this would imply that two opposing arrows of time would have emerged from the Big Bang, which would account in turn for the maintenance of time-reversal symmetry despite the ensuing dissipative nature of the universe. We would happen to live in one of them, where dissipation and entropy increase are common experience, but unaware of the existence of the other alternative possibility.”

hoppp 2y ago

When can we send information back in time? I just want to send myself some stock market data. Im pretty sure I can encode all information I need in 1 qbit(buy or sell).hehe

deleted 2y ago

[deleted]

cjfd 2y ago

The standard answer is that apparently the initial state of our universe had a low entropy and that has been increasing ever since.

throwaway314155 2y ago
That's the standard answer to a question you're not responding to, as far as I can tell.

steveBK123 2y ago

Better headline - physicists discover you could theoretically "Do Tenet".

pingou 2y ago

It seems to me that time could run backward and it would not make any difference to an observer, and the universe could alternate between running backward or forward and that would be the same to us, cause and effects would be reversed but it would only be visible to an outside observer, that has somehow time running in a different direction.

pwr22 2y ago
There's a Philip K Dick story that covers this type of thinking, "Your appointment will be yesterday".

janpmz 2y ago

How long will it take for people to realize, they are looking at a contradiction?